Various arc detection systems have been used in an attempt to detect arcing in the wiring of both residential and commercial buildings. It is recognized that arcs occurring between line and neutral or ground conductors can produce temperatures well above the ignition level of many common flammable materials, and therefore pose a significant fire hazard. For example, wiring with degraded or aged insulation and worn power cords in a home may arc sufficiently to start a fire.
Two basic types of arcing which are likely to occur in buildings are line fault arcing and contact arcing. An arc due to a line fault results from either a line-to-line short or a line-to-ground short. When a fault of this type occurs, the fault will draw current up to or beyond the capacity of the circuit, lights will dim indicating an excessive load is being drawn, and, if the associated circuit is protected by a circuit breaker, the circuit breaker will trip, interrupting the supply of power to the arc. Typically, these events will occurs in less than a second, and the arcing will have a duration insufficient to radiate or conduct substantial heat energy to the surrounding materials. Thus, the likelihood of fire is relatively low.
Contact arcing is commonly caused by use of worn electrical outlets, plugs and switches. As a result of such wear, these devices have insufficient current-carrying capacity for their originally intended purposes. For example, a plug may be subjected to a substantial number of heating cycles caused by resistance heating. This cycling gradually decomposes the insulating material around the contact area until material partially flows into the contact area thereby preventing proper electrical contact from being made at the plug. This process may become regenerative as the initial arcing produces more heat, thus, carbonizing the insulation, producing an insulating layer of the contact surfaces of the plug.
Other common sites of arcing are oxidized terminals, oxidized and worn switch contacts, and the interfaces between aluminum wiring and connection terminals made of dissimilar metals such as copper. The arcing which normally occurs under these circumstances is usually referred to as series arcing.
In view of the potential for arcing in the electrical systems of buildings, and the potential for devastating results thereof (i.e. building fires), a number of systems have been developed to detect arcing in building wiring. For example, U.S. Pat. No. 5,223,795, issued to Frederick K. Blades on Jun. 29, 1993, discloses a method and apparatus for detecting arcing in electrical connections by monitoring high frequency noise. The apparatus monitors high-frequency noise on the associated power line characteristic of arcing. One way the apparatus determines if arcing is present is to discriminate arcing noise from other high-frequency noise by determining if gaps in the noise occur every half-cycle of the power frequency.
Systems such as that disclosed in the '795 patent may operate effectively to detect arcing, but it would be desirable to improve the arc detecting capability of such systems so that the system can discriminate between different types of arcing and provide the ability to produce different responses to different types of arcing. Improvements in the performance can take a number of forms such as refining programming and refining interface circuitry to apply more accurate representations of signals monitored by the system. However, regardless of the quality of the system for monitoring and analyzing signals, the system will only produce arc protecting results as good as the systems ability to produce signals representative of the high frequency currents on the wiring being monitored. For example, current transformers are typically used to produce signals representative of the high frequency currents (desired signals) on the wiring being monitored. However, current transformers may incorporate signals representative of noise radiated onto the wires being monitored. In addition, a current transformer may also sense standing waveforms unrelated to the high frequency currents.
Accordingly, there is a need for improved sensors and circuitry for producing signals representative of the high frequency currents on the wiring being monitored by an arc detection system. In particular, there is a need for an improved sensor for use in the detection of series, line-to-neutral, line-to-ground and contact arcs which reduces or eliminates the sensing of standing waveforms.